The political barriers that divide ASEAN today are a consequence of the geographic barriers scattered throughout Southeast Asia. The Malay Archipelago is composed of thousands of tiny islands inhabited by a variety of ethnic groups. Steep mountains and dense forests disrupt transportation among them, isolating the region's ethnically, religiously and linguistically diverse populations from one another. Continental Southeast Asia, meanwhile, is split down the middle by several river systems running north to south, severing the connections between the region's eastern and western halves. The core of Southeast Asia stretches from peninsular Malaysia, through Singapore, to the island of Java. It includes the metropolises of Greater Kuala Lumpur, Singapore, Batam, Johor and Jakarta, as well as the rest of Java, which alone accounts for roughly a quarter of ASEAN's collective GDP.